Automatic attenuation circuit



July 1, 1941. F, BARR ETAL 2,247,468

AUTOMATIC ATTENUATION CIRCUIT Filed May 11, 1940 d. 5 A j INVENTOR.

flf'fw 6/7 y M a a my Patented July 1, 1941 AUTOMATIC ATTENUATIONCIRCUIT Forrest L. Barr and Carl W. Ulrich, Chicago, Ill., assignors ofthirty per cent to Robert J. Thorn, forty per cent to said Ulrich, andthirty per cent to said Barr, all of Chicago, Ill.

Application May 11, 1940, Serial No. 334,566

Claims.

This invention relates in general to communication circuits and moreparticularly to a vacuum tube circuit for the automatic attenuation ofaudio frequency transmission in a communication line.

A long felt need for a simple automatic means for controlling the levelof audio frequency transmission where transmitted signals must notexce'ed predetermined limited values in order to maintain proper qualityof reproduction in given output circuits is fulfilled by this invention.

Amplified audio frequency transmission originating from microphones orrecordings often exceeds normal transmission limits. Prior to thisinvention several means were employed to regulate and govern thetransmission of audio frequency transmission, including a manualattenuation control and automatic vacuum tube control circuits. Themanual method of control is unsatisfactory inasmuch as the attenuationis dependent entirely upon the human element and the heretofore usedautomatic vacuum tube circuits introduced undesirable amplitudedistortion in the transmitted signals.

In the present invention the means employed is entirely automatic in itsoperation and may be adjusted to accurately limit transmission topredetermined maximum levels and the automatic attenuation is effectedwithout the introduction of amplitude distortion.

The principal object of the invention is the provision of vacuum tubeand circuit means for automatically limiting the maximum level of audiofrequency transmission in a communication circuit.

A further object of the invention is the provision of an automaticallyvariable resistance network for controlling the levels of audiofrequency transmission in transmission lines.

A further object of the invention is the inclusion of vacuum tube platecircuits as the control elements in a transmission line.

A further object of the invention is the provision of vacuum tubesincluded in a circuit net work in a transmission line under thecontrolling influence of a secondary vacuum tube network.

Referring to the drawing:

Fig. 1 is a schematic diagram of the attenuation network in atransmission line.

Fig. 2 is a schematic diagram of a secondary network adapted to controlthe attenuation. network shown in Fig. 1.

LI, L2, L3, and L4 represent the input and the output of a section of atransmission line.

LI and L2 are connected to the primary of a conventional audio inputtransformer Tl. L3 and L4 are connected to the secondary of aconventional audio output transformer T2.

Conventional five element vacuum tubes VI and V2 comprise the activeelements in the attenuation or compression circuit for controlling thepower level in the transmission line.

Although pentode tubes are used in this preferred form, it is understoodthat other types of vacuum tubes may be used for the same purpose in thepresent circuit. For purposes of simplicity it is assumed that electricpower is supplied to the cathodes I and 2 of the tubes.

The secondary of the input transformer TI is connected to one side ofthe limiting resistors 3 and 4 by conductors 5 and 6 respectively. Theremaining side of the limiting resistor 3 is connected to a couplingcondenser 'l and the plate 8 of vacuum tube VI and to one side of theprimary of output transformer T2 by conductor 9. The remaining side oflimiting resistor 4 is connected to one side of the coupling condenserIll, the plate ll of the vacuum tube V2 and to the remaining side of theprimary of the output transformer T2 by conductor [2.

The center tap l3 of the secondary of the transformer TI is suppliedwith B power by conductor M, to be hereinafter described.

The center tap I5 in the primary of the output transformer T2 isconnected to resistor l6 by conductor l1. One side of by-pass condenseri8 is connected to conductor I1 and the remain ing side of the condenserconnected to ground at G. The remaining side of resistor I6 is connectedto conductor l4 by conductor IS.

The control grid 20 of the vacuum tube VI is connected to a commonjunction of one side of the bias resistor 2| at its junction with thecoupling condenser l. The control grid 22 of vacuum tube V2 is connectedto the common junction of the bias resistor 23 at its junction with oneside of condenser IS. The two grid bias resistors 2| and 23 areconnected together at 24 with conductor 25, which conductor runs tocontrol apparatus to be hereinafter described.

The suppressor grids 26 and 21 of tubes VI and V2 are connected to oneside of their respective cathodes I and 2 by conductors 28 and 29respectively. Conductor 29, which is common to cathode 2 and suppressorgrid 21, is connected to control apparatus to be hereinafter described.

One side of the output line L4 is connected to ground at G and the otherside of the output line L3 is connected by conductor 33 to the controlcircuit, to be hereinafter described.

It is apparent from the foregoing that the circuit included between theinput transformer TI and the output transformer T2 in efiect is acontinuation of the transmission line having two branches, the upperbranchconductor 5, resistor 3 and conductor 9, and the lowerbranchconductor 6, resistor 4 and conductor I2. The central or commonleg of the branch circuit is completed through the center tap I3 of theinput transformer and the center tap I5 of the output transformerthrough conductor I'I', resistor I6 and conductor I4.

It is common practice in such circuits to control the transmission levelby means of an L pad resistance network in each branch, which usuallyconsists of a series and parallel resistance. In this case the L padnetwork of the upper transmission line branch is represented by resistor3 in series with the line and a shunting or parallel resistance efiectedby the plate circuit of the vacuum tube VI between its cathode I and theplate 8 to the conductor 9. The same is true for the lower branch inthat the resistor 4 represents the series resistance and the shunt orparallel resistance iseffected by the plate circuit of the tube V2between the cathode 2, plate I1 and conductor I2. The means employed forautomatically varying the plate impedance or resistance of the tubes VIand V2 will be hereinafter described.

Three vacuum tubes are utilized in a secondary circuit which is adaptedto control the foregoing attenuation circuit. Vacuum tubes V3 and V4 areconventional triodes and vacuum tube V5 is a full wave rectifier. Forthe purpose of simplicity, the cathode heating circuit to all tubes hasbeen omitted. Line L5 is connected to the positive side of a source ofhigh voltage direct current. Line L6 is connected to the negative sideof the same source of high voltage direct current and is also connectedto ground at G. A voltage dividing resistor 51 is connectedtherebetween.

Grid 3I of vacuum tube V3, Fig. 2, is connected to output line L3, Fig.l, by conductor 30. The plate 32 of vacuum tube V3 is connected to oneside of plate resistor 33 and blocking condenser 34. One side of thecathode is connected through bias resistor 35 and by-pass condenser 31to ground at G. Plate voltage is supplied to the remaining side of theplate resistor 33 from the direct current source L5 through conductor38.

Transformer T3 is used for coupling the vacuum tube V3 to the rectifiertube V5. Inary lead of the transformer is connected to blockingcondenser 34 by conductor 39. The re maining side of the primary isconnected to ground by conductor 40.

The outside terminals of the secondary of transformer T3 are connectedto the anodes 4I and 42 of rectifier tube V5 by conductors 43 and 44respectively. The resistors 69 and I0 and the condenser II, whichconstitute a network connected across the anodes 4! and 42 of the vacuumtube V5, are for the purpose of regulation only and are not important inthe general operation of the circuit. One side of the cathodes 45 and 46are connected through bias resistor 41 and its by-pass condenser 49 toconductor 48, which conductor is connected to the center tap of thesecondary of the transformer T3 and also to the cathode of the vacuumtube V4. The grid 50 of the regulator'vacuum tube V4 is connected to thenegative side of a bias battery 5I by conductor 52.

One pri- The positive side of the bias battery is connected to one sideof both cathodes of the vacuum tube V5 by conductor 53. The plate 54 ofvacuum tube V4 connects through an on-oii switch 55 to a variable point5'3 of I 1e voltage divider 51 by conductor 58.

Conductor is also connected to the screen grids 59 and Ci of the vacuumtubes VI and V2,

ig. l, which is conventional practice in pentode type tubes. One side ofthe cathode E3 of vacuum tube V3 is connected through control resistorBI and a parallel connected by-pass condenser E".- to ground at G. Thesame side of the cathode of tube V4 is connected to the bias resistors2I and 23 at their junction Fig. 1, through conductor B2.

Conductor 29, running from the common connections to the cathodes I and2 of the tubes VI and V2, Fig. 1, is connected to a variable point ofthe voltage divider 51.

One side of milliemmeter E6 is connected to conductor i4, 1, and theremaining side of the milliammeter is connected through conductor 61,Fig. 2, to a point (58 of the voltage divider 51.

For the purpose of describing the operation of the entire circuit, itwill be assumed that the oathodes of all tubes are supplied with properoperating current and that a variable audio frequency signal isimpressed upon transformer TI through the input lines LI and L2. It isapparent that the signal will be induced in the secondary of transformerTI and transmitted through the upper and lower legs of the controlcircuit and the common center tap circuit of the transformer to theoutput transformer T2 and thence to the continuation of the transmissionline L3 and L4. The electron emission from the cathodes I and 2 of thevacuum tubes V I and V2 to their respective plates 8 and II willcomprise a shunt resistance across the two legs 9 and I2 of thetransmission line in addition to the relatively small additional shuntcircuit through the coupling condensers I and I0 and the grid resistors2i and 23.

In order to provide a non-inductive L pad network, the resistors 3 and 4are fixed at a nominal predetermined value to provide a givenattenuation in connection with the shunt path provided by the plateimpedance of the vacuum tubes VI and V2. The signal is transmittedthrough the upper and lower legs of the control circuit to the plates 8and II of the vacuum tubes VI and V2 and through coupling condensers Iand I I to their respective control grids simultaneously. Since theapplied signal is impressed simultaneously to the control grids and theplates of the vacuum tubes VI and V2 respectively, the shunt resistanceis provided in the path of the signal between the points 8 and II, andinstead of being equal to the sum of the alternating current plateresistances, it is equal to th sum of the A. C. plate resistancesdivided by the amplification constant of the tubes Vi and V2. Themilliammeter 66 indicates the plate current of the vacuum tubes VI andV2 and normally remains at zero. It will follow that the normallyattenuated signal will be impressed by conductors 9 and i2 upon theoutput transformer T2 and thence transmitted to the lines L3 and L4.

Since the vacuum tubes VI and V2 and their associated network controlthe transmission line, the following will describe the vacuum tubenetwork which controls the action of the tubes VI and V2 and thusprovides the compression or attenuation to the transmitted signal whennormal levels are exceeded.

The output audio frequency signals in the lines L3 and L4, Fig, 1, areimpressed upon the grid of the vacuum tube V3, Fig. 2, by conductor 30connecting the output line L3 and the ground connection common to lineL4 and the vacuum tube V3. Plate current is supplied to vacuum tube V3from line L5, Fig. 2, conductor 38 and plate resistor 33. Thus it willbe seen that the high impedance grid circuit of vacuum tube V3 will notseriously affect the output in the lines L3. and L4 but the signaltransmitted therethrough will be amplified by vacuum tube V3 andimpressed upon the primary of the coupling transformer T3 through thecondenser 34 and conductor 39, and condenser 37 and conductor 40.

The amplified output signal in transformer T3 is impressed upon therectifier tube V through the conductors 43 and 44 connected to theanodes 4| and 42 of the tube respectively and the midtap, completing acircuit to the cathodes of the tube through conductor 48, condenser 49and the resistor 2?.

The rectified components of the signal impressed upon the vacuum tube V5are carried. to the amplifier tube V4 through conductor 53, bias battery5i, grid conductor 52 and the cathode conductor Q8.

Under normal operating conditions where signals passing through thetransmission line, shown in Fig. l, are not in excess of a predeterminednominal value, the battery 5| biases the grid of tube V4 sufficiently tonormally reduce the plate current to zero value, hence, under normalconditions, no current will flow through resistor BI Plate voltage issupplied from a point 56 of the voltage divider 51 through conductor 58through the closed on-off switch 55 to the plate 54 of the tube V4.

The voltage drop across the resistor 6I between the cathode and groundof the tube V4, when plate current is caused to flow, is impressedthrough conductor 62 to both grids of the vacuum tubes VI and V2, Fig.1, by virtue of the common junction 24 of both the grid bias resistors2I and 23 of the vacuum tubes VI and V2 and the return through thecathodes of the tubes VI and V2 through conductor 29 to point 65 ofvoltage divider 51 and thence to the common ground G.

Condenser E4 is provided to by-pass audio frequency currents and topermit only uni-directional currents to flow in conductor 62 and toregulate the time period associated with the limiting action of thecircuit. The condenser 49 and resistor 41 also are arranged to regulatethe time period of the limiting action of the circuit. Thus it will beseen that the plate currents in vacuum tubes VI and V2 will be zero solong as the voltage of the bias battery 5I is not exceeded by therectified output potential of the vacuum tube V5. When an audiofrequency current is transmitted through the network and impressed uponthe output lines L3 and L4 of a value greater than a predeterminednormal level the rectified potential in the vacuum tube V5 will overcomethe normal bias on the grid 50 produced by the battery 5| and reduce thenormal bias to a point where plate current will flow in the tube V i andthrough the control resistor GI. The increases in plate current flow forexcessive signals will be in proportion to the level of the originaltransmitted signal. Thus a lesser negative bias will be applied to thegrids of the vacuum tubes VI and V2 which will inherently permit alarger plate current to flow in both tubes by virtue of the conductor 62which will apply the voltage drop across the control resistor 6| to thecommon junction of the grid bias resistors 2| and 23 of the vacuum tubesVI and V2. As a result of the increase in the positive bias of the tubesVI and V2 the resultant larger plate current will provide a lowerimpedance path through each tube and hence act as a lower value shuntresistance across the lines '9 and I2,

which will automatically reduce the transmitted signal to apredetermined value in the output lines L3 and L4. The milliammeter 65will automatically indicate the attenuation or suppression by indicatingthe plate currents in vacuum tubes VI and V2 in excess of normal.

Having described our invention, we claim:

1. An automatic attenuation circuit for an audio frequency transmissionline comprising input and output means, a circuit network includedbetween said input and output means, a vacuum tube means included insaid circuit network, the plate and grid circuits of said vacuum tubemeans connected as a shunt path across the said circuit network,including an additional vacuum tube circuit connected and responsive toaudio frequency signals in the said transmission line, said additionalcircuit adapted to control and automatically decrease the normalimpedance of the aforesaid vacuum tube means when signals in excess ofpredetermined values are impressed upon said transmission line to limitthe amplitude of signals in the said output means of the saidtransmission line to a predetermined level.

2. In an automatic attenuation circuit for an audio frequencytransmission line, an input transformer, an output transformer, acontrol circuit connecting said input and output transformer having twobranches, said circuit comprising series and parallel resistanceelements, a Vacuum tube means connected in each said branch of saidcontrol circuit, the plate and grid circuits of said vacuum tube meansshunt connected in each said branch in said control circuit, anauxiliary regulation circuit network including vacuum tubes responsiveto audio frequency signals in the aforesaid output transformer andconnected wit-h and adapted to control the impedance of the aforesaidvacuum tube means to govern and limit to a predetermined level audiofrequency signals transmitted in the aforesaid transmission line.

3. In an automatic attenuation circuit for an audio frequencytransmission line, an input transformer, an output transformer, acircuit network having two branches connecting said input transformerwith said output transformer, fixed resistance in series with eachbranch of said circuit, a pair of vacuum tubes connected in said circuithaving a common grid terminus, regulation means for applying voltagevariations to the said common grid terminus in proportion to the valueof signals in the said transmission line, the plate of each said vacuumtube connected to each of the aforesaid branches, the grid of each saidvacuum tube connected through condenser means to each of the aforesaidbranches, the plate and grid circuits of said vacuum tubes adapted tovary the shunt impedance path across each of the aforesaid branches inproportion to voltage variations impressed by the said regulation meansupon the said common grid terminus of said vacuum tubes to govern andlimit to a predetermined level audio frequency signals transmitted inthe aforesaid transmission line.

4. In an automatic attenuation network for an audio frequencytransmission line, a control circuit means including controlling vacuumtubes interposed in said transmission line, the plate and grid circuitsof said controlling vacuum tubes comprising a shunt impedance pathacross the said transmission line when said tubes are activated, thesaid vacuum tubes including a, common grid circuit responsive to appliedvoltages, a vacuum tube regulating means responsive to audio frequencysignals in the said transmission line, said regulating means having afixed bias voltage applied to its output tube, said output tube adaptedto control the aforesaid common grid circuit when audio frequencysignals in the said transmission line exceed a predetermined value togovern and limit to a predetermined level audio frequency signalstransmitted in said transmission line.

5. An automatic attenuation circuit for an audio frequency transmissionline comprising input and output means, a circuit network having twobranches connecting said input means with said output means, a pair ofvacuum tubes having grid and plate elements connected in said network,said tubes having a common grid terminus, the value of the internalimpedance of said tubes dependent upon the value of voltage applied tothe said common grid terminus, the plate of each said vacuum tubeconnected to each of the aforesaid branches, a condenser connecting thegrid and plate of each of the aforesaid vacuum tubes, a regulation meansresponsive to audio frequency signals transmitted in said transmissionline connected to said grid terminus, said vacuum tubes adapted to varythe shunt impedance path across each of the aforesaid branches inproportion to voltage variations impressed upon the said grid terminusby the said regulation means to control and limit to a predeterminedlevel audio frequency signals transmitted in the said output means.

FORREST L. BARR. CARL W, ULRICH.

